Up to now, as the electron-emitting devices, there have been known a hot cathode element and a cold cathode element. As the cold cathode element of those elements, there have been known, for example, a surface conduction type electron-emitting device, a field emission element (hereinafter referred as “FE type”), a metal/insulating layer/metal type emission element (hereinafter referred to as “MIM type”), etc.
As the surface conduction type electron-emitting devices, there have been known, for example, an example disclosed in Radio Eng. Electron Phys., 10, 1290 (1965) by M. I. Elinson, or other examples which will be described later.
The surface conduction type emission element utilizes a phenomenon in which electron emission occurs by allowing a current to flow into a small-area thin film formed on a substrate in parallel to a film surface. As the surface conduction type emission element, there have been reported a surface conduction type emission element using an SnO2 thin film by such as the above-mentioned Elinson, a surface conduction type emission element using an Au thin film [G. Dittmer: “Thin Solid Films”, 9,317 (1972)], a surface conduction type emission element using an In2O3/SnO2 thin film [M. Hartwell an C. G. Fonstad: “IEEE Trans. ED Conf.”, 519(1975)], a surface conduction type electron-emitting device using a carbon thin film [“Vapor Vacuum,” Vol. 26, No. 1, p 22 (1983), by Hisashi Araki, et al.], etc.
As a typical example of those surface conduction type emission elements, a plan view of the above-mentioned element structure by M. Hartwell and others is shown in FIG. 29. In FIG. 29, reference numeral 3001 denotes a substrate, and reference numeral 3004 denotes an electrically conductive film that is made of a metal oxide formed through sputtering. The electrically conductive film 3004 is formed in an H-shaped plane as shown. An electrifying process called “electrification forming” which will be described later is conducted on the electrically conductive thin film 3004 to form an electron emission portion 3005. In FIG. 29, an interval L is set to 0.5 to 1 [mm], and W is set to 0.1[mm]. For convenience of showing in the figure, the electron emission portion 3005 is shaped in a rectangle in the center of the electrically conductive thin film 3004. However, this shape is schematic and does not faithfully express the position and the configuration of the actual electron emission portion.
In the above-mentioned surface conduction type emission elements including the element proposed by M. Hartwell, et al., the electron emission portion 3005 is generally formed on the electrically conductive film 3004 through the electrifying process which is called “electrification forming” before the electron emission is conducted. In other words, the electrification forming is directed to a process in which a constant d.c. voltage or a d.c. voltage that steps up at a very slow rate for example about 1 V/min is applied to both ends of the electrically conductive film 3004 so that the electrically conductive film 3004 is electrified, to thereby locally destroy, deform or affect the electrically conductive film 3004, thus forming the electron emission portion 3005 which is in an electrically high resistant state. A crack occurs in a part of the electrically conductive film 3004 which has been locally destroyed, deformed or affected. In the case where an appropriate voltage is applied to the electrically conductive thin film 3004 after the above electrification forming, electrons are emitted from a portion close to the crack.
Examples of the FE type have been known from “Field Emission” of Advance in Electron Physics, 8, 89 (1956) by W. P. Dyke and W. W. Dolan, “Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum cones” of J. Appl. Phys., 47,5248 (1976), by C. A. Spindt, etc.
As a typical example of the element structure of the FE element, FIG. 30 shows a cross-sectional view of the elements made by the above-mentioned C. A. Spindt, et al. In this figure, reference numeral 3010 denotes a substrate, 3011 is an emitter wiring made of an electrically conductive material, 3012 is an emitter cone, 3013 is an insulating layer, and 3014 is a gate electrode. The element of this type is so designed as to apply an appropriate voltage between the emitter cone 3012 and the gate electrode 3014 to produce electric field emission from a tip portion of the emitter cone 3012.
Also, as another element structure of the FE type, there is an example in which an emitter and a gate electrode are disposed on a substrate substantially in parallel with the substrate plane without using a laminate structure shown in FIG. 30.
Also, as an example of the MIM type, there has been known, for example, “Operation of Tunnel-Emission Devices,” J. Appl. Phys., 32,646 (1961) by C. A. Mead, etc. A typical example of the element structure of the MIM type is shown in FIG. 31. FIG. 31 is a cross-sectional view, and in the figure, reference numeral 3020 denotes a substrate, 3021 is a lower electrode made of metal, 3022 is a thin insulating layer about 100 [Å] in thickness, and 3023 is an upper electrode made of metal about 80 to 300 [Å] in thickness. In the MIM type, an appropriate voltage is applied between the upper electrode 3023 and the lower electrode 3021, to thereby produce electron emission from the surface of the upper electrode 3023.
The above-mentioned cold cathode element does not require a heater for heating because it can obtain electron emission at a low temperature as compared with the hot cathode element. Accordingly, the cold cathode element is simpler in structure than the hot cathode element and can prepare a fine element. Also, in the cold cathode element, even if a large number of elements are disposed on the substrate with a high density, a problem such as heat melting of the substrate is difficult to occur. Further, the cold cathode element is advantageous in that a response speed is high which is different from the heat cathode element which is low in the response speed because it operates due to heating by the heater.
For the above-mentioned reasons, a study for applying the cold cathode elements has been extensively conducted.
For example, the surface conduction type emission element has the advantage that a large number of elements can be formed on a large area since it is particularly simple in structure and also easy to manufacture among the cold cathode elements. For that reason, a method in which a large number of elements are arranged and driven has been studied for example, as disclosed in JP-A-64-31332 by the present applicant.
Also, as the application of the surface conduction type emission element, for example, an image display device, an image forming apparatus such as an image recording device, a charge beam source, and so on have been studied. In particular, as the application to the image display device, there has been studied an image display device using the combination of the surface conduction type emission element with a phosphor that emits light by irradiation of an electron beam as disclosed in for example U.S. Pat. No. 5,066,883 by the present applicant, JP-A-2-257551, and JP-A-4-28137. In the image display device using the combination of the surface conduction type electron-emitting device with the phosphor, the characteristic superior to the conventional other image display devices is expected. For example, even as compared with the liquid crystal display device which has been spread in recent years, the above image display device is excellent in that no back light is required because it is of the self light emitting type and the angle of visibility is broad.
Also, a method in which a large number of FE type elements are disposed and driven is disclosed in, for example, U.S. Pat. No. 4,904,895 by the present applicant. Also, as an example of applying the FE type to the image display device, there has been known, for example, a plate type image display device reported by R. Meyer, etc. [R. Meyer: “Recent Development on Micro-Tips Display at LETI”, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6 to 9 (1991)].
Also, an example in which a large number of MIM type elements are arranged and applied to an image display device is disclosed in, for example, JP-A-3-55738 by the present applicant.
Among the image forming apparatuses using the above-mentioned emission element, attention has been paid to the flat type image display device thin in depthwise as a replacement of the CRT type image display device since the space is saved and the weight is light.
FIG. 32 is a perspective view showing an example of a display panel portion which forms a plane-type image display device, in which a part of the panel is cut off in order to show the internal structure.
In FIG. 32, reference numeral 3115 denotes a rear plate, 3116 a side wall, 3117 a face plate, and the rear plate 3115, the side wall 3116 and the face plate 3117 form an envelope (airtight vessel) for maintaining the interior of the display panel in a vacuum state.
The rear plate 3115 is fixed with a substrate 3111, and N×M cold cathode elements 3112 are formed on the substrate 3111 (N and M are positive integers of equal to or larger than 2 or more and appropriately set in accordance with the target number of display pixels). Also, the N×M cold cathode elements 3112 are wired by M row wirings 3113 and N column wirings 3114 as shown in FIG. 32. A portion made up of the substrate 3111, the cold cathode elements 3112, the row wirings 3113 and the column wirings 3114 is called “multiple electron beam source”. Also, at least in portions where the row wirings 3113 and the column wirings 3114 cross each other, an insulating layer (not shown) between both of the wirings is formed to keep electric insulation.
A lower surface of the face plate 3117 is formed with a fluorescent film 3118 formed of a phosphor on which phosphors (not shown) of three primary colors consisting of red (R), green (G) and blue (B) are separately painted. Also, black material (not shown) are disposed between the respective color phosphors which form the fluorescent film 3118, and further a metal back 3119 made of Al or the like is formed on a surface of the fluorescent film 3118 on the rear plate 3115 side.
Dx1 to Dxm and Dy1 to Dyn and Hv are electric connection terminals with an airtight structure provided for electrically connecting the display panel to an electric circuit not shown. Dx1 to Dxm are electrically connected to the row wirings 3113 of the multiple electron beam source, Dy1 to Dyn are electrically connected to the column wirings 3114 of the multiple electron beam source, and Hv is electrically connected to the metal back 3119, respectively.
Also, the interior of the above airtight vessel is maintained in a vacuum state of about 10−6 Torr, and there is required means for preventing the deformation or destruction of the rear plate 3115 and the face plate 3117 due to a pressure difference between the interior of the airtight vessel and the external, as a display area of the image display device increases. In a method of thickening the rear plate 3115 and the face plate 3116, not only does the weight of the image display device increase, but also a distortion of an image or a parallax occurs when viewing the display device from an oblique direction. On the contrary, in FIG. 32, there is provided a structure support (called “spacer” or “rib”) 3120 which is formed of a relatively thin glass substrate for supporting the atmospheric pressure. With this structure, a space of normally sub mm to several mm is kept between the substrate 3111 on which the multiple beam electron source is formed and the face plate 3117 on which the fluorescent film 3118 is formed, and the interior of the airtight vessel is maintained in a high vacuum state as described above.
In the image display device using the display panel as described above, when a voltage is applied to the respective cold cathode elements 3112 through the vessel external terminals Dx1 to Dxm and Dy1 to Dyn, electrons are emitted from the respective cold elements 3112. At the same time, with the application of a high voltage of several hundreds [V] to several [kV] to the metal back 3119 through the vessel external terminal Hv, the above emitted electrons are accelerated and made to collide with an inner surface of the face plate 3117. As a result, the phosphors of the respective colors which form the fluorescent film 3118 are excited and emit light, thus displaying an image.
The display panel of the above-described image display device suffers from the following problems.
Because a high voltage of several hundreds V or more (that is, a high electric field of 1 kV/mm or more) is applied between the multiple beam electron source and the face plate 3117 in order to accelerate the emitted electrons from the cold cathode elements 3112, there is a fear that a creeping discharge occurs on the surface of the spacer 3120. In particular, in the case where the spacer electric charge is produced by hitting parts of electrons emitted from the vicinity of the spacer 3120 to the spacer or sticking ions ionized by the action of the emitted electrons onto the spacer, there is the possibility that discharge is induced.
In order to solve the above problem, there has been proposed that a fine current is permitted to flow in the spacer to remove electric charge (JP-A-61-118355 and JP-A-61-124031). In the proposal, a high resistant thin film is formed on a surface of the insulating spacer as, to thereby allow the fine current to flow on the surface of the spacer. The high resistant film used in this proposal is formed of a tin oxide film, a mixed crystal thin film of tin oxide and indium oxide, or a metal film.
In order to further enhance the function of the high resistant film, an electrically conductive film is disposed on a surface of the spacer 3120 which is in contact with the substrate 3111 or the fluorescent film 3118 and in the vicinity of that surface. As a result, electric connections between the high resistant film and the substrate 3111 and the fluorescence film 3118 are ensured.
On the other hand, when a high voltage is applied between the substrate 3111 and the fluorescent film 3118, the above electrically conductive film is liable to become cause of the discharge. The discharge suddenly occurs during image display, resulting in such problems that not only the image is disturbed, but also the cold cathode element 3112 in the vicinity of the discharge portion is remarkably deteriorated, thereby being incapable of normally displaying the image after that.
The present invention has been made to overcome the above drawback of the conventional spacer, and therefore an object of the present invention is to provide an image display device which prevents discharge during image display, thereby being capable of obtaining an excellent display image.